Microorganisms such as bacteria and fungi are omnipresent in our living space and populate the most different types of surfaces. Many microorganisms are pathogens and, consequently, their spreading and/or control play(s) a special part in public health and hygiene. If such microorganisms get into our body, they may be the cause of life-threatening infections. If one contracts such an infection in a hospital, this is called a nosocomial infection.
It is started from the assumption that the Euro amounts in the two-digit billion range for the elimination of damage caused by nosocomial infections are required per year worldwide. Consequently, the control of pathogenic microorganisms plays a special part in public health and hygiene.
Apart from the warding off and/or the killing of undesired microorganisms by means of antibiotics, for instance, prophylactic measures, e.g. the creation of spaces that are hostile to the life of microorganisms are gaining more and more importance. Among these prophylactic measures the use of silver as an additive to organic and inorganic materials over the past few years has quickly gained in importance. Here, silver ions interfere with important functions of microorganisms. Nowadays, it is proceeded from the assumption that silver ions block enzymes and prevent their vital transport functions in the cell. Further effects include the impairment of the structural strength of cells and/or also a damage to the membrane structure. These effects may result in a cell damage and/or cell death. Silver has a very broad active spectrum against multi-resistant germs, as well. Small doses are sufficient for achieving a long-term effect. This is called an oligodynamic effect. Organic compounds are still added in some cases in order to increase the effectiveness of silver. It is always of importance that sufficient silver ions are present. Consequently, a nanoscale silver powder, so-called nanosilver, is used in order to achieve a large particle surface.
Silver does not have any toxic side effects in a broad dosage spectrum. Only highly increased accumulations of silver in the body may result in an argyria, an irreversible, slate-gray discoloration of skin and mucous membranes. In addition, increased silver concentrations may cause taste disorders, disorders of the sensitivity to smell and cerebral convulsions.
Moreover, it must be mentioned that, in general, the interaction between nanoscale particles and the human organism has not been sufficiently investigated as yet. Broad investigation programs have only been started recently. The antimicrobial effectiveness of silver is not sufficient for many applications. The effectiveness is only given up to 0.25 of molar saline solution. Beyond that, the formation of silver chloride takes place. The essential disadvantage of the use of nanosilver resides in a non-satisfactory cost situation. On the one hand, this is due to the high price of silver and, on the other hand, the processing of silver to nanoparticles is time-consuming and expensive. A further problem arises in the processing of nanosilver due to the formation of agglomerates, aggregates and clusters. Due to this, the active surface is reduced and, as a further consequence, the antimicrobial effect. In order to prevent this, nanosilver is deposited on particle surfaces of a carrier, e.g. TiO2, which, in turn, increases the production costs.
Consequently, there has been no lack of attempts at detecting an antimicrobial, oligodynamic and disinfecting effect in other metals. Copper, for instance, also has a strong antimicrobial effect, but at a too high zytotoxicity. The result of a search in the Internet encyclopedia Wikipedia under the search word oligodynamics is that, so far, this effect has been found in the following metals, in a descending order in accordance with their effectiveness: mercury, silver, copper, tin, iron, lead and bismuth. Gold and osmium, two noble and expensive metals, have this effect, as well.
However, it is required for many applications that, in addition to a sufficient antimicrobial effectiveness, the active substance does not have any zytotoxicity and thrombogeneity and is in general biocompatible. Active substances such as mercury, bismuth or copper do not have these properties due to their high zytotoxicity and the non-given biocompatibility.
A plurality of patent and non-patent literature deals with the production and the use of nanosilver. Further metals and inorganic compounds are only described in isolated cases. U.S. Pat. No. 5,520,664 discloses a catheter made of plastic. Atoms are introduced by means of ion implantation to achieve an antimicrobial effect. Silver, chromium, aluminum, nickel, tungsten, molybdenum, platinum, iridium, gold, silver, mercury, copper, zinc and cadmium are mentioned as a metal with an antimicrobial effect. However, only silver and copper are mentioned in the examples and special embodiments.
JP 2001-54320 describes a plastic material which contains 0.005 to 1% by weight of a mixture of molybdenum trioxide and silver oxide. The invention relates to a film consisting of an antibacterial resin component and of components which may be used for the material of partitions of clean rooms, for the uppermost layer of floor coverings, linings, briefcases, desk pads, tablecloths, packaging bags, textiles and the like. Here the problem is that in the case of the incorporation of an inorganic, antibacterial active substance the transparency of the plastic material, e.g. of vinyl chloride resin, is lost. The loss of transparency is avoided by the admixture of oxide of the hexavalent molybdenum. This application readily reveals that, upon the exceeding of the weight ratio of molybdenum trioxide to silver oxide of 95:5, an antibacterial effect can no longer be achieved. Consequently, no antibacterial effect is attributed to molybdenum oxide per se. If the ratio of molybdenum oxide to silver oxide is less than 30:70, i.e. with small shares of molybdenum oxide, a discoloration of the vinyl chloride resin takes place.
An antimicrobial plastic material is described in JP 2001-04022, which contains both organic components with an antimicrobial effect and metallic components. Silver, platinum, copper, zinc, nickel, cobalt, molybdenum and chromium are mentioned as a metallic component with an antimicrobial effect. However, silver and copper are again only mentioned as being active in the examples and preferred embodiments.
A glaze for ceramic components is disclosed in JP 2000-143369, which contains silver molybdate. Here, 0.01 to 1% of silver molybdate are added and converted to metallic silver. The effect is increased by the addition of 10 to 50% of titanium oxide.
An antimicrobial effect can also be achieved by means of components with a photooxidative effect. Due to this, reactive free radicals are formed, which damage the microorganisms. JP 11012479 describes an antimicrobial plastic material which contains an organic and an inorganic component. Metallic particles such as silver, zinc and copper and further compounds such as calcium zinc phosphate, ceramics, glass powder, aluminum silicate, titanium zeolite, apatite and calcium carbonate are mentioned as an example of inorganic components. Here, metal oxides such as zinc oxide, titanium oxide or molybdenum oxide act as a catalyst for the photooxidative effect. Consequently, JP 11012479 reveals that the antimicrobial effectiveness is only achieved, if the photooxidative mechanism occurs, i.e. the prerequisite for the effectiveness is the action of electromagnetic radiation.